To improve the overall efficiency of a gas turbine engine, a heat exchanger or recuperator can be used to provide heated air for the turbine intake. The heat exchanger operates to transfer heat from the hot exhaust of the turbine engine to the compressed air being drawn into the turbine. As such, the turbine saves fuel it would otherwise expend raising the temperature of the intake air to the combustion temperature.
The heat of the exhaust is transferred by ducting the hot exhaust gases past the cooler intake air. Typically, the exhaust gas and the intake air ducting share multiple common walls, or other structures, which allow the heat to transfer between the two gases (or fluids depending on the specific application). That is, as the exhaust gases pass through the ducts, they heat the common walls, which in turn heat the intake air passing on the other side of the walls. Generally, the greater the surface areas of the common walls, the more heat which will transfer between the exhaust and the intake air. Also, the more heat which is transferred between the exhaust and the air, the greater the efficiency of the heat exchanger.
As shown in the cross-sectional view of FIG. 1a (FIG. 1a), one example of this type of device is a heat exchanger 5, which uses a shell 10 to contain and direct the exhaust gases, and a core 20, placed within the shell 10, to contain and direct the intake air. As can be seen, the core 20 is constructed of a stack 26 of thin plates 22 which alternatively channel the inlet air and the exhaust gases through the core 20. That is, the layers 24 of the core 20 alternate between channeling the inlet air and channeling the exhaust gases. In so doing, the ducting keeps the air and exhaust gases from mixing with one another. Generally, to maximize the total heat transfer surface area of the core 20, many closely spaced plates 22 are used to define a multitude of layers 24. Further, each plate 22 is very thin and made of a material with good mechanical and heat conducting properties. Keeping the plates 22 thin assists in the heat transfer between the hot exhaust gases and the colder inlet air.
Typically, during construction of such a heat exchanger 5, the plates 22 are positioned on top of one another and then compressed to form the stack 26. Since the plates 22 can separate if not held together, the compression of the plates 22 ensures that there are always positive compressive forces on the core 20 to hold the plates 22 in place.
Applying a high pre-load to the stack 26 reduces the potential for separation of the plates 22. However, to be able to apply pre-loads to the stack 26, a pre-load assembly or support structure 50 positioned about the stack 26, is needed. In addition to applying the pre-load to the stack 26, the support structure 50 carries any additional loading exerted by the stack 26. Such additional loads can come from a variety of sources, including thermal expansion of the stack 26 and the pressurization of air (or other medium) in the stack 26.
The support structure 50 collectively includes strongbacks 40, tie rods 30, and the shell 10. The tie rods 30 are held to the strongbacks 40 by fasteners 36 positioned at the ends 32 of the tie rods 30. Because the support structure 50 supports the core 20 (namely the stack 26) and is not a heat transfer medium, the components of the support structure 50 are made of much thicker materials than those of the core 20. Unfortunately, these thicker materials cause the support structure 50 to thermally expand at a much slower rate than the quick responding core 20, with its thin plates 22. The thickness (and thus the thermal response) of the support structure 50 will also be affected by the amount of the pre-load applied to the core 20.
Differential expansion or contraction between the core 20 and the support structure 50 can result from a variety of sources, including differential thermal expansion rates and air (or fluid) pressure variations. Differential expansion or contraction between elements of the heat exchanger 5 can occur in any dimension, and typically in all dimensions at the same time. That is, not only will the core 20 expand or contract along its length, LA1, quicker than the support structure 50 will, but it also deforms faster along its width, WA1, and depth (not show).
As can be seen in FIG. 1a, to bring air into the core 20, an air inlet tube 23 is positioned within an inlet manifold 25. Likewise, an air outlet tube 29 is positioned within an outlet manifold 27. However, as the core 20 expands, or contracts, along its width (and depth) faster than that of the support structure 50, the inlet manifold 25 and outlet manifold 27 will move, as shown in FIG. 1b (FIG. 1b) (showing the core 20 differentially expanded). With the core 20 expanded (to a width of WA2), the inlet manifold 25 and the outlet manifold 27 are no longer aligned with the respective openings of support structure 50. The misalignment of the manifolds places stresses on the tubes 23 and 29, and may result in the tubes being deformed (as shown), displaced and/or otherwise damaged. Movement of the tubes 23 and 29 may cause them to contact and damage the interior portions of the core 20. Damage to the core 20 and the tubes 23 and 25 can be costly and time consuming to correct. Further, deformation of the tubes 23 and 25, can result in a disruption and a reduction of the airflow through the core 20, which in turn, can lower the efficiency of the heat exchanger 5. Also, a reduction of the air passing through the core 20, may cause severe damage to the core 20 due to overheating.
Approaches to preventing damage from lateral expansion of the core 20 have included attempts to restrain the expansion and/or contraction of the core by application of additional compressive forces. However, such expansion/contraction restraining has resulted in the core and the support structure being put under excessive loading. This loading can result in high stresses and thermal damage or failure to both the core and the support structure. Such thermal damage includes creep and/or buckling of the associated structures.
While the structures of the heat exchanger can be enlarged to carry greater loads, doing so results in certain disadvantages. These disadvantages include: a lowering of the heat transfer characteristics of the core, an increase in the differential expansion/contraction between the core and the support structure, and an increase in the cost and weight of the heat exchanger.
One approach to accommodate the width-wise differential thermal expansion and contraction has been to use an inlet bellows 60 and an outlet bellows 70, as shown in FIG. 2 (FIG. 2). The inlet bellows 60 and the outlet bellows 70 are used to keep the inlet tube 23 connected to an external inlet duct and the outlet tube 29 connected to an external outlet duct as the core 20 moves relative to the support structure 50. As the core 20 expands in width, the inlet bellows 60 and the outlet bellows 70 both deform to maintain pathways for the flow of air.
This prevents stresses from being placed on the tubes 23 and 29, as well as on the core 20.
One problem with the use of the bellows is that the outlet bellows 70 is very expensive and difficult to manufacture. This is because the outlet bellows 70 must be able function under the extreme temperatures associated with the outlet side of the core 20. Typically, these temperatures are very close to, or the same as, the temperature of exhaust gases, which enter the core 20 just after exiting from the attached turbine engine (not shown). Materials which can withstand these temperatures and continue to be sufficiently flexible over time are very expensive and difficult to use in fabricating the outlet bellows 70.
An additional problem with using a bellows system such as that shown in FIG. 2, is that with repeated thermal cycling, the core 20 can migrate about relative to the support structure 50. This can result in restrictions in the airflow, damage to the bellows, and/or failure of one or both of the bellows. Also, such core movement requires that the length of the bellows be increased, which in turn increases the cost of the heat exchanger.
Therefore, a need exists for a heat exchanger which accommodates differential expansion or contraction between the core and the supporting structure, such that the airflow through the core is not significantly disrupted. The heat exchanger must be configured to prevent failures or damage caused by buckling, creep or any other similar source. Further, the heat exchanger should be relatively simple in construction and operation to minimize its cost, weight and complexity.